Matrix acidizing is a process used to increase the production rate of wells in hydrocarbon reservoirs. It includes the step of pumping an acid into an oil- or gas-producing well to increase the permeability of the formation through which hydrocarbon is produced and to remove some of the formation damage caused by the drilling and completion fluids and drill bits during the drilling and completion process.
In order to predict the outcome in the field of the pumping of an acid, or of acid stages, into a reservoir, engineers go through a design process, which can be divided into several steps. In the first step, for example, core flood experiments are carried out, where different acids are injected, for testing, into cylindrical rock cores under various conditions. During such tests, many parameters can be varied, such as an injection rate Q, a temperature T, an acid formula Ac, and a rock type Ro.
In the core flood experiment, as acid flows into the rock, it dissolves part of the rock matrix and increases the overall permeability of the core with time. Depending on the combination of the above parameters, the dissolution pattern inside the rock can vary between face dissolution (also known as compact dissolution), wormholing dissolution and uniform dissolution. Face dissolution corresponds to the regime where acid flows so slowly that it dissolves the rock through the rock face only, located at the interface between the acid and the core. This interface moves slowly in the flow direction as more and more rock gets dissolved with time. Wormholing dissolution happens when acid flows faster than in the face dissolution regime and not all the acid is spend at the rock face. Live acid enters the core and, due to instable dissolution fronts, fingers of live acids propagate into the rock forming structures known as wormholes. If acid is pumped fast enough for the amount of acid spent during the residence time of the fluid into the core is very small, then, the acid concentration is constant within the rock and the matrix is dissolve in a uniform way. These three known dissolution regimes give rise to different acid efficiencies. Acid efficiency is measured as the amount of acid that is required by the rock core to increase its permeability to a pre-set value kw, for instance 100 times larger than the initial permeability k0 of the sample. The smaller this volume of acid is, the higher the efficiency is. The moment at which this target value of permeability increase is reached is called the breakthrough time, t0. The corresponding volume of acid is called the breakthrough volume, V0.
The measure of pore volumes to breakthrough, denoted ⊖0, (i.e. the breakthrough volume divided by the pore volume of the core PV, where PV is the volume of fluid that can be contained in the core, within the pore network), and its use to predict acid performance during a treatment job has been known to the industry for a long time. For example, pore volume to breakthrough has widely been used as a measure of the velocity at which wormholes propagate into the formation, under various conditions such as mean flow-rate Q, temperature T, rock-type Ro, and acid formulation Ac.
In order to measure pore-volume to breakthrough, acid is pumped at a constant rate Q and the pressure drop Δp across the core is monitored. The initial pressure drop when the acid reaches the inlet core face is called Δp0. When non-self diverting acids such as hydrochloric and acetic are used, as acid flows into the core, the pressure drop declines, mostly linearly. When Δp is virtually equal to 0 (i.e., the core permeability has reached a value kw orders of magnitude larger than the initial permeability k0) the pore-volume injected is recorded as the pore-volume to breakthrough ⊖0.
Recently, acid systems have been developed with the goal of achieving maximum zonal coverage in heterogeneous reservoirs. Such fluids are designed to self-divert into lower permeability zones of the reservoir after having penetrated and stimulated higher-permeability zones. When such systems are pumped using the same procedure as the one described above, the pressure drop Δp across the core may evolve in a very different way as for non-self diverting acids: the pressure does not decline linearly with time and might increase significantly over a certain period of time.